1. Technical Field
The present disclosure relates to the field of laser printers, and more particularly to determining and generating a horizontal synchronization signal for a laser printer having a multi-facet rotating mirror.
2. Related Art
The printing process of a laser printer typically includes pressing or rolling a photoreceptor drum over a sheet of paper or other print media to transfer an image from the photoreceptor drum to the print media. In some laser printers the image is first transferred from the photoreceptor drum to a rotating belt, and then from the belt to the print media. In either case, an emitter emits successive laser beam signals to “write” successive lines of the image on the photoreceptor drum as it rotates.
In most laser printers, a system of lenses and mirrors direct the laser beam signals from the emitter to, and across, the photoreceptor drum. Each successive laser beam signal writes a line of static charge, corresponding to a line of the image, on the photoreceptor drum. The emitter emits each laser beam signal based on respective streams of image line data received from an image processor. Successive lines are written to the photoreceptor drum as it rotates and as the emitter receives a stream of image line data for each line. In a color printer, there may be several rotating drums, each receiving a respective laser beam signal from a respective emitter by way of the system of lenses and mirrors.
A color tandem laser printer has multiple (e.g., four) photoreceptor drums in line for transferring respective print colors to the print media, either directly or by way of belts. A color tandem laser printer may have one or more multi-facet, polygonal rotating mirror(s) for directing multiple laser beam signals, from multiple emitters (e.g., four), to the (respective) photoreceptor drums. Each emitter receives its own stream of image line data for turning its laser beam on and off as the laser beam sweeps across the respective photoreceptor drum. To print an image accurately, the beginning of each laser beam signal must be aligned with the beginning (i.e., edge) of a print line when the emitter is receiving the image line data. In this regard, the streams of image line data that are communicated to each emitter are initiated based on respective timing signals (sometimes referred to as “horizontal synchronization signals”) received from respective horizontal synchronization signal (“horizontal sync”) sensors that are provided for each photoreceptor drum.
In most cases, each horizontal sync sensor is mounted on a circuit board near the beginning edge of its respective photoreceptor drum. As the mirror rotates, the laser beam is reflected off of a facet of the rotating mirror and sweeps toward the horizontal sync sensor and the photoreceptor drum. The horizontal sync sensor provides a horizontal synchronization signal when it is struck by the laser beam. The horizontal synchronization signal indicates that the laser beam is a known distance from the write area of the photoreceptor drum. Image line data to the respective emitter is initiated a predetermined time interval (corresponding to the known distance) after the horizontal synchronization signal is triggered.
In a color tandem laser printer more than one facet of a mirror may be used to reflect/direct laser beams to respective (e.g., 4) photoreceptor drums. Because there may be variations between the relative positions, angles, and/or lengths of the mirror facets (i.e., an imperfect multi-facet mirror), respective horizontal sync sensors and supporting circuitry are provided for each photoreceptor drum so that each horizontal synchronization signal is provided at the right time for writing a line of the image to the correct location on the photoreceptor drum. An improved approach is desirable.